The present invention relates to low temperature fired ceramics especially useful in the manufacture of electronic components or parts and further, useful in other various applications, such as heat-resistance industrial articles, tablewares, kichen utensils and decorative articles.
Recently, there has been a growing demand for a more densely integrated electronic circuits with the increasing trend toward miniaturization of computers or electronic devices of various equipment or facilities for public-utility. In such circumstances, substrates must satisfy the following requirements.
(1) Low cost PA0 (2) Light weight PA0 (3) High mechanical strength PA0 (4) The substrates have a high thermal conductivity, thereby allowing rapid radiation of generated heat. PA0 (5) In order to increase the density of a two-dimensional wiring, wiring with a conductor width of 150 .mu.m or finer can be performed. PA0 (6) The substrate sheets can be readily stacked in a multilayer configuration and thereby permit a three-dimensional wiring with an increased density. PA0 (7) Passive parts, such as inductors, resistors or capacitors, can be integrally incorporated into the substrate during stacking the substrate sheet in a multilayer configuration. PA0 (8) In order to minimize an electrostatic capacity between conductors, thereby to realize a rapid response to signals, their dielectric constant must be as small as possible. PA0 (9) For the requirement (8), conductive materials with a low electrical resistance, for example, Ag, Ag-Pd, Cu, Au, can be employed. PA0 (10) The dielectric layers have a small coefficient of thermal expansion near that of a Si semiconductor with a thermal expansion coefficient of 3.5.times.10.sup.-6 /.degree.C. to be packaged thereon, thereby permitting a direct bonding of semiconductor chips thereto. PA0 (11) Electrical resistivity between wires printed thereon is high. PA0 (12) They are not adversely affected by temperature, humidity or other atmospheric conditions and have high stability properties. PA0 (1) The substrates are subjected to warpage and cracking during repeated soldering or dip brazing operations because of their poor heat resistance, low strength and unfavorably excess thermal expansion coefficient of the order of 50.times.10.sup.-6 /.degree.C. and further their electrical resistivity will be detrimentally affected due to elevated temperature exposure. PA0 (2) When elements, such as resistors or semiconductor IC chips, which tend to generate heat, are packaged on the conventional organic resin substrates, various special consideration or designs, for example, an enlarged bonding area for the elements or the use of radiating sheets, are required in order to prevent the elements from being heated up beyond acceptable temperature levels. PA0 (3) In the fabrication of multilayer structures, it is very difficult to form fine conductors with a width of 150 .mu.m or finer or numbers of through holes with a diameter of smaller than 200 .mu.m. PA0 (4) Reliability in heat or humidity stability is low. PA0 (1) They are manufactured at a very high temperature of 1600.degree. to 1700 .degree. C. in an atmosphere of hydrogen and such special conditions considerably increase production cost compared with the former organic resin multilayer substrates. PA0 (2) Since W or Mo is employed as conductive materials, resistance of the conductors is high. PA0 (3) They exhibit a large density (from 3.8 to 3.9 g /cm.sup.3) as compared to that of the organic resin subtstrate (2.0 g/cm.sup.3). PA0 (4) Their dielectric constant (9 to 10 at 1 MHz) is high as compared to the dielectric constant of the former organic substrates (3 to 5 at 1 MHz). PA0 (5) Although they exhibit an appreciably lowered thermal expansion compared with the former organic substrate, their thermal expansion coefficient of 7.0.times.10.sup.-6 /.degree.C. (from room temperature to 250.degree. C.) is too large as compared to that of Si semiconductor.
Heretofore, for the above mentioned purposes, various organic multilayer substrates or alumina ceramic multilayer substrates have been employed. However, these conventional substrates are superior in some properties, but in other properties, they are inferior. Therefore, any substrate heretofore available can not give satisfaction in all respects and presents problems or difficulties in practical applications.
More specifically, in the case of organic multilayer substrates, circuits are formed on copper cladding layers bonded onto both sides of phenol resin or epoxy resin substrate sheets, then the resin sheets are bonded to form a multilayer configuration by using epoxy resin adhesives and through holes are formed between the conductors by a mechanical perforating process. However, the organic resin multilayer substrates thus fabricated have the following problems:
On the other hand, multilayered alumina substrates have the following disadvanteges:
In view of the above circumstances, there have been proposed several low temperature fired glass ceramic compositions.
As examples of low temperature fired ceramics, low temperature fired ceramics using crystallized glass are presented in some reports. For example, for the purpose of obtaining lower temperature fired ceramics with a high strength, MgO-Al.sub.2 O.sub.3 -SiO.sub.2 base composition containing additives of B.sub.2 O.sub.3 and nucleating agent are fired at a temperature of 900.degree. to 1000 .degree. C. to precipitate crystallites of cordierite in the resulting ceramics.
For the same purpose, there have been reported as other examples lower temperature fired ceramics produced from Li.sub.2 O-Al.sub.2 O.sub.3 -SiO.sub.2 base compositions with additives of B.sub.2 O.sub.3 and nucleating agent. In this composition, in order to increase the strength of the fired products, spodumene is precipitated. In general, in order to make ceramics at lower firing temperatures, glass phase is required to be contained in much larger quantities. However, in the case, it is very difficult to achieve a high strength of 1800 kg/cm.sup.2 or higher. In either the case of MgO-Al.sub.2 O.sub.3 -SiO.sub.2 or Li.sub.2 -Al.sub.2 O.sub.3 -SiO.sub.2, low temperature fired ceramics with a high strength are attained by precipitating high strength crystals therein by heat treating. However, in the cases of these known ceramic compositions, the aimed crystals do not precipitate below 900.degree. C. and the compositions are in a glassy phase in the vicinity of 500.degree. C. to about 800.degree. C. Thus, when fine conductor patterns are printed onto green ceramic tapes and fired simultaneously together with the ceramics, the printed patterns tend to flow and deform. Therefore, great difficulty will be presented in the formation of circuits with a high precision. Further, another difficulty may be encountered in firing process. When green ceramic tapes containing a large amount of an organic binder are fired to remove the organic binder component, it is requested to fire the tapes so that the organic binder does not remain as carbon in the glass phase and, thus, firing should be carried out without softening the glass and while maintaining the porosity required to facilitate degassing. For such requirement, a heating rate in the firing step must be about 2.degree. C. per minute and, as shown in FIG. 1, it takes eight hours to heat up 950.degree. C.
Further, there is reported low temperature fired ceramics using a mixture of a noncrystallized glass and insulating or refractory oxides. The examples are a borosilicate glass (B.sub.2 O.sub.3 -SiO.sub.2) containing additives of refractory materials, such as cyanite or anorthite, or a mixture of lead borosilicate glass (B.sub.2 O.sub.3 -SiO.sub.2 -PbO-Al.sub.2 O.sub.3) and insulating oxides, such as forsterite and ZrO.sub.2. These ceramics also, as in the case of low temperature fired ceramics using crystallized glasses previously set forth, can not successfully form circuits in a high resolution due to liability to flowing of patterns and consume a very long firing time of 12 to 18 hours to heat to 900.degree. C. due to a slow heating rate of the order of 1.degree. C. per minute, as shown in FIG. 1. Further, in the cases of the noncrystallized glasses, since it is impossible to obtain a required strength level only from the noncrystallized glasses, refractory materials are added to the noncrystallized glasses in order to achieve an acceptable strength level in the composite systems of the noncrystallized glasses and the added refractory materials. However, the noncrystallized glass phase does not crystallized even in a final firing stage at 800.degree. to 1000.degree. C. The noncrystallized glass phase softens during firing and causes deformation of patterns. Further, softening of the glass makes degassing very difficult and, thus, the binder component is required to be slowly driven off by firing for a very long time at temperatures below the softening temperature of the glass. FIG. 2 shows shrinkage curves of various low temperature fired ceramics. As can be seen from this figure, in both cases of crystallized glass and noncrystallized glass with addition of refractory materials, shrinkage due to softening of glass (noncrystallized or crystallized glass) is observed at temperatures of 200.degree. to 700.degree. C.